90 ELECTRON-MICROSCOPIC STRUCTURE OF PROTOZOA 



upon which studies of ultrastructure may be expected to throw 

 significant light include the relationships of such intracellular 

 forms to the host cells, and the morphological basis of a peculiar 

 gliding locomotion without any visible means of propulsion, 

 characteristic especially of gregarines. 



Within the Subphylum Sporozoa, the Class Gregarinomorphea 

 consists of relatively large symbiotes in the digestive tracts of 

 invertebrates; often they have an intracellular phase early in the 

 life cycle but typically they are extracellular at maturity. A thick, 

 longitudinally striated pellicle is characteristic, and considerable 

 internal differentiation is evident by light microscopy. The 

 organisms examined to date with the electron microscope are all 

 members of the Order Eugregarinida, and only vegetative 

 individuals have been described. The most complete morpholo- 

 gical studies are those by Beams, Tahmisian, Devine, and 

 Anderson (1957, 1959a) on Gregarina rigida, by Kiimmel (1958) on 

 species of Gregarina and Beloides, and by Grasse and Theodorides 

 (1957, 1958, 1959) on species of Gregarina, Stylocephalus, and 

 Lophocephalus. Shorter accounts are by Theodorides (1959), Klug 

 (1959), and Lacy and Miles (1959). 



In most of the gregarines studied the body of the adult vegeta- 

 tive cell is covered with longitudinal crests or ribs about 1 fx high, 

 formed by orderly folding of the cell surface (only in Apolocystis, 

 examined by Lacy and Miles, the surface bears long, stiff hairs). 

 Two limiting membranes are present, the inner one typically 

 appearing thicker, the outer one often wrinkled. Both of these 

 cover the pellicular folds. At least on the outer extremities of the 

 crests and perhaps on their sides as well, fine fibrils run longitudi- 

 nally or obliquely within the inner membrane (Fig. 32, PI. IX). A 

 third layer, thicker, less well-defined and variously reported as 

 membranous or filamentous, forms a smooth, unfolded sheet 

 running along the base of the crests in most micrographs but 

 apparently is sometimes lacking. In the cytoplasm within the 

 crests, Grasse and Theodorides (1959) observed in some species 

 compact groups of very fine fibrils. 



Adhesion of potential gametocytes may occur in these 

 gregarines, sometimes quite early in the vegetative phase. Pairs 

 of organisms pictured in this state by Beams and colleagues show 



